1. Field of the Invention
The present invention generally relates to kerfing propellant sticks and, more particularly, is concerned with an apparatus and method for making spaced pairs of transaxial kerfs in propellant sticks without increasing stick length or reducing stick density.
2. Description of the Prior Art
Making transaxial kerfs, or cuts, in lengths of propellant stick containing multiple axial perforations has been practiced for many years, as attested by U.S. Pat. No. 660,568 to Gathmann which issued Oct. 30, 1990. The term "transaxial" means the kerf is made transverse or generally perpendicular to the longitudinal axis of the propellant stick. The purpose for transaxially kerfing axially perforated propellant sticks is to create an escape route for products of combustion which otherwise become trapped in the perforations during deflagration of the propellant. Without the presence of the kerfs, pressures in the perforations will build up and likely cause fracturing of the stick, adversely affecting burn rates.
The Gathmann patent discloses transaxial kerfs made in the propellant stick from one side. More recently, transaxial kerfs arranged in axially spaced pairs emanating from opposite sides have been provided in perforated propellant sticks. The amount or pressure build-up in the perforations of the propellant stick is controlled by the amount of spacing between the pairs of transaxial kerfs. Further, the transaxial kerfs of each pair terminate at inner ends being located beyond the longitudinal axis of the stick such that the kerfs overlap one another at their inner ends. The overlap of the kerf inner ends provides assurance that all perforations have been transected and vented. Also, the transaxial kerfs of each pair are axially offset from one another, with the amount of axial spacing between the kerf pairs typically being greater than the amount of offset between individual kerfs of each pair. The amount of kerf offset is selected so as to ensure that the kerfed propellant sticks will retain a degree of relative strength or rigidity sufficient to withstand the required subsequent handling operations.
The propellant of the kerfed sticks is, in effect, granular propellant specifically arranged to allow for maximum pack density. Maximum pack density is highly desired when loading cartridge ammunition with a fixed chamber volume to maximize the loaded propellant charge weight. From increased propellant charge weights increased internal ballistic pressures are produced which, in turn, generate increased muzzle velocities. In a kinetic energy application, the impact energy is directly proportional to the square of the projectile velocity. The performance of advanced development penetrators depends greatly on the velocity with which they can be delivered to the target.
One conventional method for making transaxial kerfs in propellant sticks involves the use of a hydraulic or mechanical press to drive offset blades into a stationary stick of propellant from its opposite sides to prescribed depths to maintain overlap of adjacent kerfs. The spacing between transaxial kerf pairs is established either by indexing the propellant stick between each kerfing cycle of the press or by gang kerfing with several sets of blades at a time which are properly spaced.
However, both of these ways of forming axially spaced pairs of transaxial kerfs in the propellant stick have drawbacks. On the one hand, forming a single pair of transaxial kerfs at a time in the propellant stick and then indexing the propellant stick before making the next pair of kerfs yields an extremely low rate of production. On the other hand, in gang kerfing the propellant stick is typically displaced by the introduction of the blades into the propellant stick thereby creating forces which tend to deflect adjacent sets of blades. This latter effect, which increases as the number of blades in the gang are increased, generates a kerf which is not perpendicular to the longitudinal axis of the propellant stick. The angle of the kerf reduces the overlap such that the reliability of the perforations' venting may be compromised.
Furthermore, the conventional method produces a kerf in the form of a V-shaped deformation open at the surface and narrowing to an intersection at the full depth of the kerf. For the JA-2.19-perforation stick propellant used by the 120 mm tank ammunition program, the net result of this deformation is a two percent increase in the length of the stick of propellant after kerfing. This corresponds to a two percent decrease in propellant stick density, resulting in a two percent loss in loaded propellant charge weight and in projectile velocity.
Consequently, a need exists for improvement of the manner by which kerfing of propellant sticks is performed so that the above-described drawbacks associated with the conventional technique will be avoided.